1. Field of the Invention
The present invention relates to an image forming apparatus and an image forming method, particularly to an electrophotographic image forming apparatus and an electrophotographic image forming method.
2. Description of the Related Art
In an electrophotographic image forming apparatus, since the image is formed using static electricity, the image density, line width, and the print position will fluctuate due to fluctuation in environmental conditions (such as temperature and humidity of the environment where the image forming apparatus is used), time degradation of a photoreceptor, a developer and/or the like (i.e., change in durability), and therefore it is not possible to perform stable image formation.
In order to prevent such problem, a control for stabilizing the image to be formed is performed (such control is referred to as “image stabilization control” hereinafter) is performed in which information about environmental conditions, information about durability, and information about an image-adjusting pattern image are detected and fed back to the condition for forming image (referred to as “image forming condition” hereinafter) to thereby stabilize the image to be formed (see, for example, Japanese Unexamined Patent Application Publication No. 2006-39036). Here, the “image-adjusting pattern image” is a pattern image exclusively formed for adjusting image. By performing the image stabilization control, it is possible to stably form an image even if there are factors that destabilize the image formation.
Generally, it is known there are two image stabilization control methods, one is a method in which the toner density of an unfixed image-adjusting pattern image formed on an intermediate transfer belt is detected by a toner density sensor arranged opposing the intermediate transfer belt (such method is referred to as an “image stabilization control method (1)” hereinafter), and the other one is a method in which the toner density of an image-adjusting pattern image fixed onto the sheet is detected by a toner density sensor arranged in a sheet conveying section provided on the downstream side of the fixing section (such method is referred to as “image stabilization control method (2)” hereinafter).
In the image stabilization control method (1), since the toner density sensor is located on the downstream side of a secondary transfer section and arranged so as to face the intermediate transfer belt, it is not possible for the toner density sensor to detect fluctuation generated in both the secondary transfer section and the fixing section, and feed back the fluctuation to the image forming condition. In contrast, in the image stabilization control method (2), since it is also possible to detect the fluctuation generated in both the secondary transfer section and the fixing section, which can not be detected by the image stabilization control method (1), and feed back the fluctuation to the image forming condition, high image quality can be obtained compared with the image stabilization control method (1).
However, in the image stabilization control method (2), when using the toner density sensor to detect the information about the color, the density and the like of the image fixed onto the sheet, if such information is detected immediately after the sheet has passed through the fixing section, there is a possibility that, due to the influence of a phenomenon called “thermochromism”, the color may change compared with the image quality seen by the user. Here, “thermochromism” means a phenomenon in which the color of the toner changes due to heat. A technique for compensating the influence of thermochromism has been proposed (see, for example, Japanese Unexamined Patent Application Publication No. 2011-186087).